1. Field of the Invention
This invention relates to the field of making mixed metal hydroxides or layered double hydroxide compounds. More specifically, the invention relates to a high surface area synthetic meixnerite product capable of adsorbing CO.sub.2 at a high rate and method for making the same.
2. Technology Review
Naturally occurring meixnerite exists as a secondary mineral in the cracks of serpentine rocks near Ybbs, Persenberg in lower Austria. In its crystalline state, such meixnerite material is tabular, colorless and has perfect basal cleavage. Natural meixnerite is closely related to hydrotalcite and pyroaurite in overall structure. It has an infrared absorption spectrum that compares favorably with those for hydrotalcite and other synthetic magnesium-aluminum double hydroxides. Hydrotalcite ideally has the formula Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3 .multidot.4H.sub.2 O. Synthetic hydrotalcites generally have the formula Mg.sub.l-x Al.sub.x (OH).sub.2 !x+(CO3.sup.n-.sub.x/n).multidot.m H.sub.2 O where x may very over a range of approximately 0.1 to 0.35 corresponding to magnesium to aluminum ratios as high as 9 and as low as 1.9. Others in the literature have claimed to synthesize hydrotalcites with magnesium to aluminum ratios as high as 12 (see Shen et al, "Synthesis and Surface Acid/Base Properties of Magnesium-Aluminum Mixed Oxides Obtained From Hydrotalcites", Langmuir, Vol. 10, 1994). In some circles, meixnerite is even listed among other hydrotalcite-like materials, or grouped in the broader family of "hydrotalcites". Under the latter definition, meixnerite is a carbonate-free member of the hydrotalcite family which has only hydroxy anions. Still others refer to meixnerite as an all hydroxyl, layered double hydroxide.
Meixnerite, or magnesium aluminum hydroxide hydrate, is often symbolized by the formula Mg.sub.6 Al.sub.2 (OH).sub.18 .multidot.4H.sub.2 O, although still other formulaic representations include: Mg.sub.4 Al.sub.2 (OH).sub.14 .multidot.3H.sub.2 O and Mg.sub.3 Al(OH).sub.8 !OH.multidot.2H.sub.2 O. Hydrotalcite and meixnerite share the same general X-ray diffraction pattern.
The synthesis of meixnerite is fairly new and the following various methods of manufacture do not appear to be commonly practiced or commercially practical. In March 1980, G. Mascolo et al described a synthesis process in Mineralogical Magazine whereby magnesium oxide, decomposed from basic magnesium carbonate at 650.degree. C. for 6 hours, was combined with an alumina gel and rotated in an air thermostated oven for one week at 80.degree. C. The resulting product was then dried over silica gel. It was analyzed to contain some brucite compound and about 0.8-1.0 wt. % carbon dioxide.
Six years later, I. Pausch et al wrote of a variation on the aforementioned process in Clay and Clay Minerals. Therein, magnesium oxide, annealed at 1050.degree. C., was combined with an alumina gel(.delta.-Al.sub.2 O.sub.3), MgC.sub.2 O.sub.4 .multidot.2H.sub.2 O and distilled water. This combination was heated to between 100.degree.-350.degree. C. at a pressure of 100 MPa for various reaction times ranging from 7 to 42 days. IR spectroscopy analysis of the resulting product showed some carbonate contamination, but at an intensity of less than 5% as compared to natural hydrotalcite.
From a series of experiments reported by E. Dimotakis et al in Inorganic Chemistry, Vol. 29, No. 13 (1990), synthetic meixnerite was prepared by calcining a hydrotalcite of the formula Mg.sub.3 Al(OH).sub.8 !CO.sub.3 !.sub.0.5 .multidot.2H.sub.2 O at 500.degree. C. to form a metal oxide solution. This oxide was then hydrolyzed at 25.degree. C. in a carbon dioxide-free environment.
It is a principal objective to provide a method for making a synthetic meixnerite capable of adsorbing gases like CO.sub.2. It is another objective of this invention to provide a means for making a synthetic meixnerite which, when activated, has a B.E.T. surface area of about 290 m.sup.2 /g or greater. It is yet another objective to make a synthetic meixnerite that consists of a double layered hydroxide with 100% or more of its weight in water. It is still another objective to make a high surface area synthetic meixnerite that when activated may be capable of catalyzing chemical reactions with or without supported metals. It is still another objective to provide a process for synthesizing meixnerite and related minerals from two different hydrotalcites. It is still another objective to create a hydrotalcite-like compound having significantly lower carbonate levels and virtually no other anion contamination. It is still another objective to provide a method for making high surface area synthetic meixnerite which is not dependent on the use of alumina gels.
On a preferred basis, synthetic meixnerite can be made from fairly inexpensive and readily available reactants by this process, thus making it suitable for the commercial scale production.